An enzymatic mechanism for balancing the stoichiometry of nitrogen and phosphorus in a shallow Chinese eutrophic lake.

The over-enrichment of lake waters with nitrogen (N) and phosphorus (P) has become a serious environmental problem, but modes of change in stoichiometry and enzymatic regeneration along trophic gradients are largely unknown. Seasonal variations in the kinetics of extracellular aminopeptidase (LAP) and alkaline phosphatase (AP), together with the composition of phytoplankton and concentrations of N and P, were examined from Jun 2013 to September 2014 in a Chinese shallow lake in which two basins had contrasting trophic states. The turbid basin had a significantly higher concentration of chlorophyll a and lower ratios of N to P. In parallel, the turnover time of organic N mediated by LAP (T) was significantly shorter, and its maximum velocity (V) was significantly higher compared to those in the clear basin. Considering data from both basins, there were linear decreases in N/P and the ratios between dissolved inorganic N and total N with an increasing trophic state index, coupled with a significantly positive relationship between N/P and T. Additionally, with decreasing TN/TP, the Michaelis constant (K) of the AP increased linearly, reducing the efficiency of P regeneration. In contrast, the K value of LAP decreased, and V increased, which enhanced N mineralization by simultaneously increasing the reaction velocity and improving the affinity for substrate. Additionally, the K value of LAP was significantly related to that of AP and the ammonium concentration. Thus, substrate affinity acted as a key factor modifying the pathways of enzymatic degradation of organic N and P according to their stoichiometry in the water column. Phytoplankton composition was directly linked to T. Overall, this study seemed to be the first to connect a stoichiometric shift of N and P with kinetics of extracellular enzymes responsible for their regeneration along trophic gradients, presenting an additional pathway to overcome nitrogen deficiency in eutrophic lakes.